Petroleum well cementing involves mixing a slurry of cement, water, and other additives and pumping it down through steel casing to critical points in the oil well annulus around the casing or in the open hole below the casing string. The primary functions of the cementing process are to restrict fluid movement between geological formations and to bond and support the casing. In addition, the cement aids in protecting the casing from corrosion, preventing blowouts by quickly sealing formations, protecting the casing from shock loads in drilling deeper wells, and sealing off lost circulation or thief zones.
A common problem in petroleum well cementing is the flow of liquid from the cement slurry into porous earth formations in contact with the cement. This fluid loss is undesirable since it can result in dehydration of the cement slurry, and it causes thick filter cakes of cement solids which can plug the well bore; moreover the fluid lost can damage sensitive formations. Cement fluid loss is particularly a problem in the process known as squeeze cementing.
There is a requirement, therefore, for materials which, when added to the cement formulation, reduce the loss of fluid from the slurry to porous formations.
The use of polyvinyl alcohol as a fluid loss control additive in cements used in oil field applications is known. Polyvinyl alcohol has been used unmodified, crosslinked with various additives and grafted with other moieties as exemplified in the following references.
U.S. Pat. No. 6,180,689 entitled “Fluid Loss Control Agents and Compositions for Cementing Oil Wells Comprising said Fluid Loss Control Agent” of Moulin describes a fluid loss control agent for a petroleum industry (or analogous) cement slurry, comprising a surfactant and a micro-gel obtained by chemical cross-linking of a polyvinyl alcohol. The micro-gel and the surfactant are compatible with petroleum industry cement additives and can also produce compositions which are gas tight. The micro-gel is obtained by reacting the polyvinyl alcohol in solution with agents which can condense with at least two alcohol functions at a pH of less than 10, the molar concentration of the cross-linking agent with respect to the monomer units of the PVOH are in the range of about 0.1% to 0.5%. The condensing agent used to synthesize the micro-gel is preferably selected from glyoxal, glutaraldehyde, maleic acid, oxalic acid, dimethylurea, polyacroleins, diisocyanates, divinylsulphate, and chlorides of diacids. Glutaraldehyde is particularly preferred. The micro-gel is typically prepared in aqueous solution comprising 2% to 5% (by weight), preferably on the order of 3.5% of cross-linked polyvinyl alcohol. Cellulosic polymers are sometimes provided as liquid additives; however, these systems have several drawbacks. For one, they are provided at low concentrations—less than 10%. Moreover, cellulosics tend to retard the set time of the cement and increase viscosity of the slurry. Another drawback of cellulosics is that they tend to thin as they move down hole, causing solids to settle. Cellulosic additives are also thought more expensive than necessary for wells operating at less than about 65° C. or so. Cellulosic polymers are commonly prepared in dispersions in mineral oil or heavy brine. This serves to “close” the polymer and allow for higher concentrations than would otherwise be possible. This method is practical, but not preferred, as the salt or mineral oil may adversely affect the cement properties if not sufficiently diluted or washed away.
U.S. Pat. No. 5,728,210 entitled “Composition and Method to Control Cement Slurry Loss and Viscosity” of Moran et al. and U.S. Pat. No. 5,850,880 entitled “Composition and Method to Control Cement Slurry Loss and Viscosity” of Moran et al., utilize polyvinyl alcohol as a liquid fluid loss additive for use in cementing well bores, particularly oil and gas well bores. The liquid fluid loss additive is formed from dissolving partially hydrolyzed polyvinyl alcohol in water. The polyvinyl alcohol has a molecular weight of about 200,000. The solutions contain polyvinyl alcohol in an amount between 20 and 200 parts to 1000 parts of water. The polyvinyl alcohol is used in conjunction with a dispersing sulfonated polymer and surfactant. This combination is then adjusted to individual well conditions with chelating agent, cross-linking agent, biocides, antifoams, or combinations of these. The dispersants are polymers such as melamine sulfonate polymer, vinyl sulfonate polymer and styrene sulfonate polymer and mixtures of these. These dispersant materials are usually prepared at low pH and can be used in the acid form, or neutralized to form salts of the polymers, wherein the salt can be a Group I or Group II metal salt, or ammonium salts (common salts). The liquid solution can be used in conjunction with additional polymer dispersing agents to achieve lower viscosity cement slurries.
An aqueous gel that is formed from a polyvinyl alcohol or vinyl alcohol copolymer and a partially methylated melamine-formaldehyde resin in the presence of a pH regulating agent which provides an initial acid pH during the formulation of the gel is described in U.S. Pat. No. 5,061,387 entitled “Aqueous Gel System of Partially Methylated Melamine-Formaldehyde Resin and Polyvinyl Alcohol” of Victorius. These gel-forming compositions control the permeability of underground formations during water flooding and chemical flooding operations. These plugging techniques are also used during well workovers, for example, to plug leaks in well casings or to temporarily plug wells, in fracture treatments, to consolidate unconsolidated formations, and to correct the injection profile of a well by sealing high-permeability streaks so that flooding fluids will enter the formation in a more desirable front.
U.S. Pat. No. 5,009,269 entitled “Well Cement Fluid Loss Additive and Method” of Moran et al., relates to cementing of a casing string in a well bore, and more particularly to a fluid loss additive for addition to a cement slurry to be used in the cementing job. A fluid loss additive is provided which is effective at temperatures of up to about 95° C., has limited effect on slurry viscosity, and does not significantly retard cement setting. The additive is comprised of a partially hydrolyzed vinyl acetate polymer, calcium sulfate, a cross-linker for the polymer, and optionally a defoamer. Because of difficulties in manufacturing a PVOH with a molecular weight above about 200,000, the use of PVOH was considered limited to formation temperatures of about 50° C. This disclosure teaches that the useful temperature can be increased to about 95° C. by including cross-linking materials in the additive. In the presence of boric acid (or other cross-linker) and calcium sulfate (or other sulfates), the PVOH behaves as if it has a higher molecular weight. At temperatures much above 95° C., the cross-linked PVOH is not thermally stable.
U.S. Pat. No. 4,703,801 entitled “Method of Reducing Fluid Loss in Cement Compositions which may Contain Substantial Salt Concentrations” of Fry et al., discloses a method of reducing fluid loss in cement compositions which may contain substantial salt concentrations. The compositions are comprised of water, hydraulic cement and a fluid-loss additive comprising a graft polymer having a backbone of lignin, lignite, derivatized cellulose and various synthetic polymers such as polyvinyl alcohol, polyethylene oxide, polypropylene oxide and polyethyleneimine. The grafted pendant groups comprise homopolymers, copolymers and terpolymers of 2-acrylamido-2-methylpropane-sulfonic acid, acrylonitrile, N,N-dimethylacrylamide, acrylic acid, N,N-dialkyl-aminoethylmethacrylate and their salts. The backbone comprises from about 5 to about 95 percent by weight of the graft polymer, and the pendant groups can comprise from about 5 to about 95 percent by weight of the graft polymer.
U.S. Pat. No. 4,967,839 entitled “Method and Composition for Cementing in a Wellbore,” of Carpenter et al. discloses a cement composition for oil and gas wells comprising at least 2 weight percent of tricalcium aluminate, at least 2 weight percent of gypsum, and between 0.3 to 2.0 weight percent of a polyvinyl alcohol having a degree of hydrolysis that is less than about 92 percent. According to Carpenter, polyvinyl alcohols with a molecular weight of less than 75,000 are preferred.
Additional references of interest follow. U.S. Pat. No. 6,110,270 entitled “Method for Influencing Moisture Content and Migration in Building Materials,” of Beckenhauer teaches an aqueous PVOH solution for typical use as a coating on building materials in order to prevent the migration of moisture through a porous building. The solutions may contain from about 0.01% to about 30% by weight of PVOH which may have a molecular weight ranging from about 5,000 to about 500,000.
U.S. Pat. No. 6,739,806 entitled “Cement Compositions with Improved Fluid Loss Characteristics and Methods of Cementing Subterranean Formations,” to Szymanski et al. discloses methods for preventing fluid loss in cement slurries by connecting two polymers via a pH sensitive crosslinking agent, such as a polyvalent cation. In preferred embodiments the additive contains a first PVOH polymer with a molecular weight of at least 80,000 and a second PVOH polymer with a molecular weight of about 8,000. The polymers are dissolved in water with a cross-linker and the pH is adjusted until the solution achieves a desired molecular weight. Likewise, U.S. Pat. No. 5,594,050 to Audebert et al. discloses a fluid loss control agent which employs chemically cross-linked PVOH.
U.S. Pat. No. 5,105,885 entitled “Well Cementing Method Using a Dispersant and Fluid Loss Intensifier,” of Bray et al. discloses a fluid loss additive package containing an ethoxylate, a dispersant material and, optionally, a water soluble polymeric compound. The polymeric compound may comprise polymers such as polyvinyl alcohol or 2-acrylamido-2-methylpropyl sulfonic acid (AMPS) copolymers.
Fluid loss additives are conventionally provided in powdered form or in dilute solutions. High molecular weight additives are conventionally believed superior; however, such additives are not generally amenable to being provided in concentrated solution because viscosity becomes too high. Accordingly, conventionally fluid loss additives are supplied at concentrations less than 10 percent by weight when provided in aqueous form. This leads to large inventory requirements and high shipping costs per pound of additive since mostly water is being handled.
Alternatively, additives are provided in solid form, typically mixed with a dry cement composition which makes it difficult to adjust characteristics of the composition quickly in response to drilling requirements and leads to unnecessary additive usage and/or inferior performance. Worse, in cases where only seawater is available, such as offshore drilling installations, the additives tend to have lower solubility in seawater and may mix poorly with a seawater cement slurry.
In one aspect of the present invention, the fluid loss additives are supplied in concentrated aqueous form which enables adjustment of hydraulic cement slurry characteristics in real time in order to optimize slurry characteristics, particularly fluid loss. The additives in the concentrates of the invention are important as they are especially valuable in offshore applications where space is limited and changes need to be made quickly.